Study question: Do the basal body temperature (BBT) shift and the cervical mucus markers for the beginning of the post-ovulatory infertile phase (POIP) of a menstrual cycle agree with the corresponding urinary pregnanediol glucuronide (PdG) threshold value?
Summary answer: Perfect agreement between the cervical mucus markers and BBT shift and the hormonal definition of the start of post-ovulatory infertility occurred for only 7-17% of the cycles.
What is known already: The PdG threshold of 7.0 µmol/24 h is an objective and accurate marker for the beginning of the POIP. The rise in serum progesterone also produces the BBT shift and changes in cervical mucus which determine the mucus peak. Serum progesterone and urinary PdG are closely correlated when variations in urine volume are taken into account.
Study design, size, duration: Individual menstrual cycle profiles of urinary PdG excretion rates for 91 fertile cycles from normally cycling women were analysed to identify the day of the beginning of the POIP. These days were compared with those determined by the day of the BBT shift +2 days, the day of the mucus peak +4 days and the later of these two indicators. The study lasted 3 years.
Participants/materials, setting, methods: A total of 62 women with normal menstrual cycles were recruited from three centres: Palmerston North, New Zealand; Sydney, Australia and Santiago, Chile. The cycles were displayed individually in a proprietary database program which recorded the PdG excretion rates, the BBT shift day and the cervical mucus peak day. A group of 15 women from a separate Chilean study had PdG urinary data measured as well as their day of ovulation determined by ultrasound.
Main results and the role of chance: The BBT and cervical mucus markers differed significantly in their identification of the beginning of the POIP when compared with the PdG excretion rate of 7.0 µmol/24 h. The observation that the BBT shift day and the mucus peak day could be identified even though the PdG excretion rates were still at baseline levels in some cycles could lead to an unexpected pregnancy for women using these natural family planning (NFP) indicators.
Limitations, reasons for caution: The study consisted only of fertile cycles from women with regular cycles of 20-40 days duration. All the women were intending to avoid a pregnancy during the study, thus the limits of the fertile window were not tested.
Wider implications of the findings: The NFP signals occurring earlier than the PdG threshold day could lead to an unexpected pregnancy. The signals occurring on the same day or later than the PdG threshold would not lead to unexpected pregnancies, but would require extra abstinence that could lead to non-compliance with the NFP method. A possible improvement in reliability of NFP methods is suggested.
Study funding/competing interests: This study (project #90905) was funded by the NDP/UNFPA/UNICEF/WHO/World Bank Special Programme of Research, Development and Research Training in Human Reproduction (HRP). D.G.C. currently works for a diagnostic development company, Science Haven Ltd. The other authors have nothing to declare.
Keywords: basal body temperature; home urine test; menstrual cycle profiles; oestrone glucuronide; pregnanediol glucuronide.
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